1. Field of the Invention
This invention relates to semiconductor devices including memory transistors, and more particularly to transistor semiconductor device including field oxide structures above a well.
2. Description of Related Art
The redistribution of phosphorus (P) during thermal oxidation and the redistribution of phosphorus (P) at Field Oxide (FOX) interfaces are generally known. See Sze, "VLSI Technology", McGraw Hill Book Company, pp. 129-131 (1988). For example, a P-field implant (B+ 150 keV 4E12 ions/cm.sup.-2) was implanted through the FOX to compensate for the concentration loss. After this process, for the FOX region, the maximum boron concentration to just below the FOX region. For the .phi. active region, the maximum boron concentration is far away from the active N+ region.
U.S. Pat. No. 5,430,324 of Bencuya for "High Voltage Transistor Having Edge Termination Utilizing Trench Technology" shows multiple transistor cells with spaced apart field oxide regions rings. Bencuya forms trenches between insulating regions in order to increase breakdown voltages. However, the use of the trench isolation technique differs from the present invention which avoids the use of trench isolation structures to achieve the objective of avoiding breakdown, by effectively increasing the voltage at which breakdown would occur.
Increasing the spacing between doped regions is generally known to reduce concentrations of dopant and to raise breakdown voltages. Although providing added spacing between doped regions and use of isolation regions is generally known to raise breakdown voltages, the above prior art does not deal with application of this principle to the conventional design in which the doped source/drain regions overlap with field oxide regions to maximize density of devices in a CMOS type of device.
FIG. 2A shows a plan view layout of a device 110 in which a prior art breakdown condition can occur.
FIG. 2B is a cross-section of the device 110 shown in FIG. 2A, taken along line 2B-2B' in FIG. 2A.
As shown in FIG. 2A, the device 110 is formed on a P-doped silicon semiconductor substrate 112 on which field oxide (FOX) silicon dioxide regions 118, 118' have been formed on the left and the right of the device 110. An N-well 114 has been formed in the substrate 112 between and below the FOX region 118 and the FOX region 118'. Within the N-well 114 a P-well 116 is formed which extends beneath much of the lower surface area of the FOX regions 118, 118'. On the surface of the P-well is formed a gate electrode stack 129 of a gate oxide layer 120, a polysilicon layer 122 and a polycide (refractory metal silicide) layer 123. A set of self-aligned N+ doped source/drain regions 122/126 have been formed in the P-well 116. Source region 124 extends to the left beneath the FOX region 118. Drain region 126 extends to the right beneath the FOX region 118'. Between the FOX region 118 and the source region 124 is an overlap region 125. Between the FOX region 118' and the drain region 126 is an overlap region 127, which can be seen in greater detail in FIG. 2A where the full area of overlap is illustrated.
FIG. 2C shows the device 110 of FIG. 2A modified slightly to illustrate in somewhat exaggerated dimensions the fact that the P-well region 116 may have a modified thickness where the regions 116' have been raised by the manufacturing process to provide narrow necks 130 and 131 between the source region 124 and the drain region 126 respectively and the N-well 114.
In the device 110 of FIG. 2C, breakdown often occurs in the overlap regions 125 and 127 between the FOX regions 118 and N+ source/drain regions 122/126 due to high doping concentration of phosphorus from N+ source/drain regions 122/126. For boron concentration distribution, due to thermal oxidation during manufacturing, as shown in FIG. 4 by the chart of the ratio C.sub.S /C.sub.B vs. temperature from 900.degree. C. to 1300.degree. C. showing the effects of redistribution of boron during thermal oxidation C.sub.B =1.times.10.sup.16 cm.sup.-3, where:
C.sub.S =Surface Concentration of Phosphorus and PA1 C.sub.B =Bulk Concentration of Phosphorus.
Curves are shown for Dry oxygen (Dry O.sub.2), which is the upper curve, and for oxygen through -97.degree. C. water, which is the lower curve.
To compensate for this concentration loss, an additional -(P-F field B+ 150 keV 4E12 ions/cm.sup.2) implantation was added such that the maximum boron concentration contacts with the maximum phosphorus concentration in FOX regions 125, 127 and builds a high field. But the maximum boron concentration is far away from the bottom of drain region 126 so this field is smaller than the field in the FOX regions 125, 127.
Especially emphasizing (0.5 .mu.m flash EPROM) with a twin well arrangement with a P-well formed in an N-well, an N+ region in a buried P-Well rule. There is an extension of the N+ active area to N+ implant area: 0.5 .mu.m due to process fluctuation. The N+ implant will diffuse to the FOX region due to lateral diffusion.
The FIG. 2C rule has a value .ltoreq.0 .mu.m and has an N+ junction breakdown &lt;7 volts.
This invention can be employed in a P-well, formed in an N-substrate; in the absence of the N-well, also.
FIG. 2D shows three curves of concentration of dopant (logarithmic) in atoms/cm.sup.3, as a function of distance in micrometers with a curve for P.sup.11 and a curve for B.sup.11, up to about 10.sup.17 atoms/cm.sup.3. In FIG. 2D, there is a curve with X.sub.1, X.sub.2, and X.sub.null at about 0.4 micrometers.